Trying to write a wrapper to ImageMagick

imagemagick.cs

var histogram = GetImageHistogram(this.wand, out var lenPtr);
var len = lenPtr.ToUInt32();
var result = new Dictionary<MagickColor, int>();
try
{
    for (var i = 0; i < len; i++)
    {
        var ptr = histogram + i * IntPtr.Size;
        Console.WriteLine($"{i} iteration, ptr: {histogram}, cur: {ptr}");

        // PixelGetRed fails with assert in native code:
        // MagickWand/pixel-wand.c:1367: PixelGetRed: Assertion `wand->signature == MagickWandSignature' failed.
        var color = new MagickColor(
            PixelGetRed(ptr),
            PixelGetGreen(ptr),
            PixelGetBlue(ptr),
            PixelGetAlpha(ptr));
        result[color] = (int) PixelGetColorCount(ptr).ToUInt32();
    }

    return result;
}
finally
{
    DestroyPixelWands(histogram, len);
}

native.cs

        // PixelWand **MagickGetImageHistogram(MagickWand *wand, size_t* number_colors)
        [DllImport(libPath, EntryPoint = "MagickGetImageHistogram", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr GetImageHistogram(IntPtr wand, [Out] out UIntPtr numberColors);

        // PixelWand **DestroyPixelWands(PixelWand** wand, const size_t number_wands)
        [DllImport(libPath, EntryPoint = "DestroyPixelWands", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr DestroyPixelWands(IntPtr pixelWands, [Out] uint numberWands);

        // double PixelGetRed(const PixelWand* wand)
        [DllImport(libPath, EntryPoint = "PixelGetRed", CallingConvention = CallingConvention.Cdecl)]
        public static extern float PixelGetRed(IntPtr pixelWand);

        // double PixelGetGreen(const PixelWand* wand)
        [DllImport(libPath, EntryPoint = "PixelGetGreen", CallingConvention = CallingConvention.Cdecl)]
        public static extern float PixelGetGreen(IntPtr pixelWand);

        // double PixelGetBlue(const PixelWand* wand)
        [DllImport(libPath, EntryPoint = "PixelGetBlue", CallingConvention = CallingConvention.Cdecl)]
        public static extern float PixelGetBlue(IntPtr pixelWand);

        // double PixelGetAlpha(const PixelWand* wand)
        [DllImport(libPath, EntryPoint = "PixelGetAlpha", CallingConvention = CallingConvention.Cdecl)]
        public static extern float PixelGetAlpha(IntPtr pixelWand);

        // size_t PixelGetColorCount(const PixelWand *wand)
        [DllImport(libPath, EntryPoint = "PixelGetColorCount", CallingConvention = CallingConvention.Cdecl)]
        public static extern UIntPtr PixelGetColorCount(IntPtr pixelWand);